Halve and quarter the pineapple and remove the flesh from the skin with a sharp knife. Cut the flesh into pieces. Arrange 1 quarter of each bowl on a dessert plate, covered with pineapple pieces.
Add the remaining pieces of wetting agent. Prepare the sauce just before serving. Pour all the ingredients into a tall metal bowl and place it on a hot water bath. Stir with the whisks of the hand mixer on the highest setting until the mixture rises on the whisks and becomes thick and frothy.
Pour the hot sauce over the fruit and serve immediately, decorated with pistachios and lemon balm.